Insular Cycas micronesica Habitats Respond Similarly to Aulacaspis yasumatsui Invasion, Regardless of Co-Occurring Consumers

Abstract

The natural distribution of Cycas micronesica includes three island groups. Damage to the widespread tree from the armored scale Aulacaspis yasumatsui was initiated with the 2003 invasion of Guam and the 2007 invasion of Rota. This herbivore has threatened the unique gymnosperm species with extinction. The number and identity of co-occurring consumers are dissimilar among disjunct insular subpopulations, and six of these habitats were used to assess tree mortality trends to confirm that A. yasumatsui stands alone as the greatest threat to species persistence. Following the initial infestation outbreak of this pest into each new subpopulation, the standing seedlings and saplings were the first to be culled, the juvenile plants were the next to be culled, and then the adult trees were killed more slowly thereafter. The timing of this plant population behavior did not differ among habitats with five other consumers, three other consumers, one other consumer, or no other consumers. We have shown that A. yasumatsui acting as the sole biotic threat in an isolated subpopulation can generate a decline in survival that is as rapid as when it is acting in conjunction with up to five other consequential consumers. This armored scale is the most acute threat to C. micronesica, and adding other specialist herbivores to the scale herbivory does not alter the speed and extent of initial plant mortality.